Fully automated radiosynthesis of N¹-[¹⁸F]fluoroethyl-tryptophan and study of its biological activity as a new potential substrate for indoleamine 2,3-dioxygenase PET imaging

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• 作者：Jean Henrottin^a ; ^b ; ^{Jean.Henrottin@ulg.ac.be" class="auth_mail" title="E-mail the corresponding author} ; Christian Lemaire^a ; Dominique Egrise^c ; ^d ; Astrid Zervosen^a ; Benoit Van den Eynde^e ; Alain Plenevaux^a ; Xavier Franci^f ; Serge Goldman^c ; ^d ; André ; Luxen^a
• 关键词：Tryptophan ; PET imaging ; Indoleamine 2 ; 3-dioxygenase ; 1-(2-[18F]fluoroethyl)tryptophan ; Fully automated radiosynthesis ; FASTlab
• 刊名：Nuclear Medicine and Biology
• 出版年：2016
• 出版时间：June 2016
• 年：2016
• 卷：43
• 期：6
• 页码：379-389
• 全文大小：1455 K

文摘

Indoleamine 2,3-dioxygenase (IDO) catalyzes the initial step in the catabolism of l-tryptophan along the kynurenine pathway and exerts immunosuppressive properties in inflammatory and tumor tissues by blocking locally T-lymphocyte proliferation. Recently, 1-(2-[¹⁹F]fluoroethyl)-dl-tryptophan (1-[¹⁹F]FE-dl-Trp) was reported as a good and specific substrate of this enzyme. Herein, the radiosynthesis of its radioactive isotopomer (1-[¹⁸F]FE-dl-Trp, dl-[¹⁸F]rong class="boldFont">5rong>) is presented along with in vitro enzymatic and cellular uptake studies.

Methods

The one-pot n.c.a. radiosynthesis of this novel potential PET imaging tracer, including HPLC purification and formulation, has been fully automated on a FASTlab™ synthesizer. Chiral separation of both isomers and their formulation were implemented on a second cassette. In vitro enzymatic and cellular uptake studies were then conducted with the d-, l- and dl-radiotracers.

Results

The radiolabeling of the tosylate precursor was performed in DMF (in 5 min; RCY: 57% (d.c.), n = 3). After hydrolysis, HPLC purification and formulation, dl-[¹⁸F]rong class="boldFont">5rong> was obtained with a global radiochemical yield of 18 ± 3% (not decay corrected, n = 7, in 80 min) and a specific activity of 600 ± 180 GBq/μmol (n = 5). The subsequent separation of l- and d-enantiomers was performed by chiral HPLC and both were obtained after formulation with an RCY (d.c.) of 6.1% and 5.8%, respectively. In vitro enzymatic assays reveal that l-[¹⁸F]rong class="boldFont">5rong> is a better substrate than d-[¹⁸F]rong class="boldFont">5rong> for human IDO. In vitro cellular assays show an IDO-specific uptake of the racemate varying from 30% to 50% of that of l-[¹⁸F]rong class="boldFont">5rong>, and a negligible uptake of d-[¹⁸F]rong class="boldFont">5rong>.

Conclusion

In vitro studies show that l-[¹⁸F]rong class="boldFont">5rong> is a good and specific substrate of hIDO, while presenting a very low efflux. These results confirm that l-[¹⁸F]rong class="boldFont">5rong> could be a very useful PET radiotracer for IDO expressing cells in cancer imaging.